We explore a variational Ansatz for lattice quantum systems—named long-range entangled-plaquette state—in which pairs of clusters of adjacent lattice sites are correlated at any distance. The explicit scale-free structure of correlations built in this wave function makes it fit to reproduce critical states with long-range entanglement. The use of complex weights in the Ansatz allows for an efficient optimization of nonpositive definite states in a fully variational fashion, namely without any additional bias (arising, e.g., from pre-imposed sign structures) beyond that imposed by the parametrization of the state coefficients. These two features render the Ansatz particularly appropriate for the study of quantum phase transitions in frustrated systems. Moreover, the Ansatz can be systematically improved by increasing the long-range plaquette size, as well as by the inclusion of even larger adjacent-site plaquettes. We validate our Ansatz and its optimization procedure in the case of the XX and Heisenberg chain, and further apply it to the case of a simple, yet paradigmatic model of frustration, namely the $J_1-J_2$ antiferromagnetic Heisenberg chain. For this model we provide clear evidence that our trial wave function faithfully describes both the short-range physics (particularly in terms of ground-state energy) and the long-range one expressed by the Luttinger exponent, and the central charge of the related conformal field theory, which govern the decay of correlations and the scaling of the entanglement entropy, respectively. Finally we successfully reproduce the incommensurate correlations developing in the system at strong frustration, as a result of the flexible representation of sign (phase) structures via complex weights.

In order for the wide-band-gap Zn-IV-nitrides to find applications in power electronics or optoelectronics, controlled $n$-type doping is required. We systematically explore group-V and group-VI dopants that can act as donors in ${\mathrm{ZnGeN}}_2$ and ${\mathrm{ZnSiN}}_2$. We address compensation by native acceptors as well as self-compensation by formation of $DX$ centers. For ${\mathrm{ZnGeN}}_2$, we find that ${\mathrm{S}}_{\mathrm{N}}$, ${\mathrm{Se}}_{\mathrm{N}}$, ${\mathrm{P}}_{\mathrm{Ge}}$, and ${\mathrm{As}}_{\mathrm{Ge}}$ act as shallow donors. ${\mathrm{P}}_{\mathrm{Ge}}$ has the lowest formation energy and can be used to reach carrier concentrations up to $n=1.3\times {10}^{19}{\mathrm{cm}}^{-3}$. ${\mathrm{P}}_{\mathrm{Si}}$ acts as a shallow donor in ${\mathrm{ZnSiN}}_2$; however, because of strong compensation from native acceptors, we find that ${\mathrm{ZnSiN}}_2$ cannot be doped $n$ type.

The presence of an edge in a two-dimensional (2D) material can induce an accumulation of charges near the edge by the electric field. Similar to the 2D plasmon, the accumulated charges can oscillate collectively, forming a distinct plasmon mode, which is called edge-plasmon. In contrast to the 2D plasmon, the electric potential of the edge-plasmon is localized near the edge. In this work, we discuss the edge-plasmon in a 2D material with finite width, such as a graphene nanoribbon, and we find that the localization of the electric field to the edge induces a phase difference of $\pi /2$ between electric fields in the directions parallel and perpendicular to the edge. As a result, the induced electric field rotates on the surface and also induces rotating current, which does not exist in the 2D plasmon. The edge-plasmon can be excited by incident light, and we predict that the rotating current generates rotation of the electric field of the scattered light.

We study the eigenenergies and optical properties of both direct excitons in a phosphorene monolayer in different dielectric environments and indirect excitons in heterostructures of phosphorene with hexagonal boron nitride. For these systems, we solve the 2D Schrödinger equation using the Rytova-Keldysh (RK) potential for direct and both the RK and Coulomb potentials for indirect excitons. The results show that excitons formed from charge carriers with anisotropic effective mass exhibit enhanced (suppressed) optical absorption, compared to their 2D isotropic counterparts, under linearly polarized excitations along the crystal axis with relatively smaller (larger) effective carrier masses. This anisotropy leads to dramatically different excited states than the isotropic exciton. The direct exciton binding energy depends strongly on the dielectric environment, and shows good agreement with previously published data. For indirect excitons, the oscillator strength and absorption coefficient increase as the interlayer separation increases. The choice of RK or Coulomb potential does not significantly change the indirect exciton optical properties, but leads to significant differences in the binding energy for small interlayer separation.

The two-temperature model (TTM) is commonly used to represent the energy exchange between atoms and electrons in materials under irradiation. In this work we use the TTM coupled to molecular dynamics (TTM-MD) to study swift heavy ion irradiation of Au and W finite nanowires. While no permanent structural modifications are observed in bulk, nanowires behave in a different way depending on thermal conductivity and the electron-phonon coupling parameter. Au is a good heat conductor and it does not transfer energy from electrons to phonons too efficiently. Therefore, energy is quickly carried away from the track so that both electronic and lattice temperatures remain quite uniform across the sample at all times. W has a lower thermal conductivity and a larger electron-phonon coupling, thus supporting an inhomogeneous lattice temperature profile with temperatures well above melting lasting several picoseconds in the irradiated region. Both W and Au nanowires display radiation-induced surface roughening. However, in the case of W there is also sputtering and the formation of a hole in the central part of the wire, purely due to the energy transferred to the atoms by the electrons. The physical mechanisms underlying these findings are rationalized in terms of a combination of sputtering, vacancy formation, and melt flow phenomena. The role of the electron-phonon coupling parameter $g$ is analyzed.

We report a systematic high-pressure study of magnetic topological semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ through measurements of synchrotron x-ray diffraction (XRD), magnetization, electrical, and Hall transports combined with first-principle theoretical calculations. No evident trace of structural phase transition is detected through synchrotron x-ray diffraction over the measured pressure range of 0.2–50.9 GPa. We find that the ferromagnetism and the anomalous Hall resistivity are monotonically suppressed as increasing pressure and almost vanish around 22 GPa. The anomalous Hall conductivity varies nonmonotonically against pressure at low temperatures, involving competition between original and emergent Weyl nodes. Combined with first-principle calculations, we reveal that the intrinsic mechanism due to the Berry curvature dominates the anomalous Hall effect under high pressure.

Here, I develop a model for a two-level system that respects the time-reversal symmetry of the atom Hamiltonian and the Kramers theorem. The two-level system is formed by two Kramers pairs of excited and ground states. It is shown that due to the spin-orbit interaction it is in general impossible to find a basis of atomic states for which the crossed transition dipole moment vanishes. The parametric electric polarizability of the Kramers two-level system for a definite ground state is generically nonreciprocal. I apply the developed formalism to study Casimir-Polder forces and torques when the two-level system is placed nearby either a reciprocal or a nonreciprocal substrate. In particular, I investigate the stable equilibrium orientation of the two-level system when both the atom and the reciprocal substrate have symmetry of revolution about some axis. Surprisingly, it is found that when chiral-type dipole transitions are dominant the stable ground state is not the one in which the symmetry axes of the atom and substrate are aligned. The reason is that the rotational symmetry may be spontaneously broken by the quantum vacuum fluctuations, so that the ground state has less symmetry than the system itself.

Superconducting films are fascinating due to their tunability associated with the interfacial effects. Here we report the superconductivity of single-crystalline $\mathrm{LaS}{\mathrm{i}}_2\left(112\right)$ films epitaxially grown on Si(111) substrates, which exhibit the superconducting transition temperature $\left(T_c\right)$ of 3.2 K in films thicker than 21 monolayers (ML) in the strong-coupling regime. At the interface, the $\mathrm{LaS}{\mathrm{i}}_2$ films bear the tensile strain, leading to the formation of thickness-dependent stripes. The local superconducting gap varies in real space in accordance with the stripes. In some areas with the thickness of 10–21 ML the superconductivity is enhanced, while in films thinner than 10 ML the superconductivity is suppressed due to the suppression of electronic states at the Fermi level. Our work provides insights into the interfacial effects on the superconductivity of epitaxial compound films from both lattice and electronic aspects.

The manipulation of Rashba effects in two-dimensional (2D) electron systems in semiconductors is highly desirable for controllable electronic and optical applications. Here, combining a first-principles investigation and model analysis, we use uniaxial stress to control monolayer BiTeI as a Rashba 2D semiconductor. We find that the stress-driven electron system can be described by an effective anisotropic Rashba model including all three Pauli matrices, and uniaxial stress allows an out-of-plane spin component because of rotational symmetry breaking. When appropriate electron carriers are introduced into the monolayer, an in-plane electric field can induce a charge current and three spin current components (including that based on the out-of-plane spin) because of the reduced symmetry. Therefore, uniaxial stress can be used to control Rashba 2D electron systems such as monolayer BiTeI for promising spintronic devices.

Pressure has become an effective way to obtain new materials with desirable properties. Considering the various stoichiometries, wide applications, and unique structures of binary H-S, H-B, and B-S compounds, especially at high pressures, it is worth expecting that they might form ternary compounds with interesting properties as well. Here, phase stabilities of H-B-S ternary compounds, in the pressure range from 0 to 200 GPa, are reliably determined through first-principles unbiased structural search calculations. A hitherto unknown orthorhombic HBS compound with Ama2 symmetry is identified to be stable above 25 GPa, in which B and S atoms alternate to form distorted chains and H atoms are covalently bonded with Bs. With pressure, Ama2 HBS undergoes a semiconductor-to-metal electronic transition and even becomes superconducting. The exploration of the binary B-S phase diagram indicates that B-S compounds tend to decompose into elemental solids above 208 GPa. However, two S-rich phases, ${\mathrm{B}}_2{\mathrm{S}}_3$ and $\mathrm{B}{\mathrm{S}}_2$, are predicted at low pressures, exhibiting semiconducting and metallic properties, respectively. $\mathrm{B}{\mathrm{S}}_2$ shows a calculated $T_c$ value of 21.9 K at 200 GPa, becoming a superconductor among bulk binary B-S compounds.

Morphotropic phase boundary (MPB) and polymorphic phase boundary ferroelectric solid solutions show enhanced electromechanical response and are sought after as actuator and transducer materials. Compared to others, the lead-free piezoelectric solid solution ${\mathrm{N}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3-\mathrm{BaTi}{\mathrm{O}}_3$ stands out in two important respects: (i) low piezoelectric coefficients $(\sim 200\mathrm{pC}/\mathrm{N})$ and (ii) anomalous decrease in the depolarization temperature at the MPB, the reason for which is still unclear. Here we show that both features are related; this is caused by an increasing intervention of a nonferroelectric structural distortion which compromises the gains the system acquires at the MPB via the interferroelectric instability. The propensity of the intervening nonferroelectric distortion, identified as in-phase octahedral tilt, grows considerably as the MPB is approached causing considerable structural-polar disorder, weakening the overall strength of ferroelectric interaction.

${\mathrm{SrCo}}_2{\mathrm{As}}_2$ is a peculiar itinerant magnetic system that does not order magnetically, but inelastic neutron-scattering experiments observe the same stripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based superconductors along with evidence of magnetic frustration. Here we present results from neutron diffraction measurements on single crystals of $\mathrm{Sr}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ that show the development of long-range AF order with Ni doping. However, the AF order is not stripe type. Rather, the magnetic structure consists of ferromagnetically aligned (FM) layers (with moments laying in the layer) that are AF arranged along $\mathbf{c}$ with an incommensurate propagation vector of (0 0 $\tau$), i.e., a helix. Using high-energy x-ray diffraction, we find no evidence for a temperature-induced structural phase transition that would indicate a collinear AF order. This finding supports a picture of competing FM and AF interactions within the square transition-metal layers due to flat-band magnetic instabilities. However, the composition dependence of the propagation vector suggests that far more subtle Fermi surface and orbital effects control the interlayer magnetic correlations.

The investigation of higher-order elastic moduli of polycrystalline solids is a challenging task. For this purpose the decomposition of the polycrystal Gibbs free energy at hydrostatic pressure in terms of the finite strain tensor components, taking into account the fourth-order contributions, is given. We give the definition of the fourth-order elastic moduli for the polycrystal (the fourth-order Lamé coefficients) at an arbitrary pressure. We obtain the relationships between the Lamé coefficients of the fourth order at pressure $P$ with the corresponding constants of the single-crystal grains constituting the polycrystal. The case of the arbitrary grain symmetry and, in particular, when the grains have a cubic lattice, is considered. The calculation results for the second-, third-, and fourth-order Lamé coefficients of the series metals with cubic structure grains at $P=0$ are analyzed. For polycrystalline tungsten, the dependence of the fourth-order elastic constants on pressure in the range of 0–600 GPa is presented.